1. Field of the Invention
This invention relates to improvements in methods and circuitry for braking polyphase motors, and more particularly to improvements in methods and circuitry for creating a negative torque in a polyphase dc motor, and still more particularly in methods and circuitry for creating the negative torque on a spinning rotor of a polyphase dc motor in order to slow the rotor down quickly.
2. Description of the Prior Art
Although the present invention pertains to polyphase dc motors, in general, it finds particular application in conjunction with three phase dc motors, particularly of the brushless, sensorless type that are used for rotating data media, such as found in computer related applications, including hard disk drives, CD ROM drives, floppy disks, and the like. In computer applications, three phase brushless, sensorless dc motors are becoming more popular, due to their reliability, low weight, and accuracy.
Motors of this type can typically be thought of as having a stator with three coils connected in a "Y" configuration, although actually, a larger number of stator coils are usually employed with multiple motor poles. Typically, in such applications, eight pole motors are used having twelve stator windings and four N-S magnetic sets on the rotor, resulting in four electrical cycles per revolution of the rotor. The stator coils, however, can be analyzed in terms of three "Y" connected coils, connected in three sets of four coils, each physically separated by 90.degree..
In operation, the coils are energized in sequences in each of which a current path is established through two coils of the "Y", with the third coil left floating. The sequences are arranged so that as the current paths are changed, or commutated, one of the coils of the current path is switched to float, and the previously floating coil is switched into the current path. Moreover, the sequence is defined such that when the floating coils is switched into the current path, current will flow in the same direction in the coil which was included in the prior current path. In this manner, six commutation sequences are defined for each electrical cycle in a three phase motor.
In the past, polyphase dc motors were usually braked or stopped by two different methods. The first method is to tristate the stator coils so that no current flows through them. The rotor is allowed to spin freely and slows down due to frictional forces present in the motor. A second method is to connect all of the coils to a resistor that quickly dissipates the electromagnetic energy in the coils through the resistor. The rotor then stops more quickly than if just the frictional forces are used to slow the rotor down. These methods are relatively slow and are not controllable by the operator.